There's usually two things that cause this. The most common is under the needle plate; the feed dogs can be packed full of dust and lint and it to the extent it presses on the stitch length function. Unscrew the needle plate, clean out all fluff, take an pin or tooth pick and scrape out the grooves in the feed dogs. If this is the case you can be sure the bobbin case has to come out to be cleaned too. Carefully check the parts under the base for any thread, piece of broken neede etc.
The other is if you have a very loose stitch length lever, I think there is a screw on the side some where that can be tightened, but I have to look up the details.